The only diesel ones I know of are customized
mpguinos, i.e.
high voltage injectors (HDi diesel) or a tdi who's ECU generates a fuel consumption signal.
and possibly flow sensor based units, a-la zemco (one flow sensor for tank feed and one for return)
mileage computer for a carbureted engine
There may be some way to map the OBD rpm and lod/tps to a reasonable approximation of fuel consumption, but need to build the 3d map to do it.